1. Field of Invention
This invention relates in general to grid-like structures of the type suitable for use as collimators for shielding radiation receiving and imaging devices from the effects of distorting radiation, and more particularly to structures of the above type suitable for use with high energy, i.e. 100 to 1,000 KEV, radiation.
2. Summary of Prior Art
The use of such structures as collimators is well known as may for example be seen from the Anger camera case. This device is a special type of radiation receiver used by the medical profession to locate and judge the extent of diseased tissue within a patient's body by the creation of photograph-like images of radioactive concentrations therein. A radioactive material is injected into the patient's bloodstream or administered orally which tends to collect in the diseased tissue. Formation of an image of an object which is a radioactive source and which therefor is its own source of radiation, however, presents a situation nonanalogous to formation of an image of an object which is illuminated by common light, or even X-rays, from a separate source, as in conventional photography. In order to get a clear image of a radioactive concentration a selection must be made from the rays emanating from the concentration in all directions of those rays which will clearly produce the image. This selection may be made so as to produce an enlarged, a miniaturized, or a same-size image of the concentration, but in all cases nonselected rays must be kept from the receiver. A collimator of a radiation absorbing material such as lead has been found to perform the selection function well and is presently used with all such devices for this purpose.
The Anger camera has thus become a significant medical tool both for diagnostic purposes and as a means to facilitate surgery by decreasing exploratory time because the spatial location of the diseased area is precisely known and by assuring all diseased tissue is found because the precise extent of the diseased area is also known.
Presently the above-described units are used with radiation energy levels of about 150 KEV, and many types of collimators have been produced for this energy level which are operationally effective and relatively efficiently manufacturable. An example of one such collimator is a number of corrugated sheets of lead approximately 0.010 inch thick having flattened ridges, sealed together by epoxy cement in a ridge-to-ridge configuration. Units of this type are particularly useful in examinations using a scintillation camera. However, I have found that new medical techniques have created a demand for a collimator suitable for use at energy levels approaching 300 KEV, and above.
It is elementary that as the radiation energy level increases, the thickness of the collimator walls must also increase. Experience indicates, however, that the efficient fabrication of a collimator suitable for use with such high energy radiation is by no means elementary. Various methods have been tried, but for one reason or another each was unsatisfactory.
For example, casting the collimator as a single unit using removable pins in the mold to provide the holes has been tried. This method while producing an operational device is impractical since due to high friction between the cast lead and the pins and the fact that some collimators are convergent or divergent (to allow enlarged or miniaturized image formation) relative to the radiation source each of the pins used to create the holes must be removed individually. This process is time consuming and costly, especially when one realizes that some such collimators have 1000 or more such holes.
A second exemplary attempt was to cast thick corrugated lead sheets and assemble them as was done at low energy. This alternative also failed due in this case to joint leakage i.e. the epoxied joints are permeable to high energy radiation and since these joints are adjacent to each other in a straight line in this case too much distoring radiation reaches the receiver. Further, attempts to avoid this problem in this alternative by creating an overlap raised insurmountable technical assembly problems.